1. Field of the Invention
This invention relates to a developing apparatus, particularly a developing apparatus for visualizing latent images by the use of a one component magnetic developer.
2. Description of the Prior Art
In general, dry developing methods for developing electrostatic charge patterns can be divided into a two component developing process and a one component developing process. In the former process, the developer comprises a mixture consisting of carrier particles such as iron powder, glass beads or the like and toner particles for developing electrostatic images actually. This two component developing process has disadvantages in that changes in the ratio of carrier particles to toner particles results in variations of density between pictures and in that the deterioration of carrier particles leads to the reduction of quality in pictures.
On the other hand, the one component developing process does not have such disadvantages as in the two component developing process since it utilizes no carrier particle. Among the various developing processes, therefore, this one component developing process is highly hopeful. A certain one component developer which is generally known and used includes magnetic particles mixed with the toner particles for preventing frictional charging due to the relative movement between the particles and for obtaining a certain means which conveys the developer toward a development area opposed to electrostatic images.
The amount of the magnetic particles which are contained in the developer is limited by the fact that the toner is fixed on a transfer paper by heat, pressure or the like to form a toner image thereon. Practically, the developer includes 10% by weight to 60% by weight of the magnetic particles relative to the toner particles. However, the toner and magnetic particles are different from each other in specific gravity so that a volume occupancy of the magnetic particles in the toner will be of 20% or less. In such a small volume occupancy of the magnetic particles, it will be difficult for the toner to form long brushes of low density at a magnetic pole. When a layer of toner is limited in thickness on a toner carrying member to a few milimeters, it is easy for to the toner layer to become uneven.
It is easy for the unevenness of the toner layer on the carrying member to show up in the developed images. Moreover, if a dense layer of toner is uneven in thickness, the toner particles may be agglomerated when they are pressed against the surface of a photoconductive member which is an electrostatic image bearing member. Such an agglomeration may damage the photoconductive member. In the one component developing process, therefore, it is required to form thin layers of toner having even thickness on the toner carrying member.
In order to control the thickness of particle layer on the carrying member, a thickness limiting member can be generally used to form a narrow slit between the carrying member and the limiting member. When the carrying member is moved relative to the thickness limiting member, a toner layer which can be actually obtained thereby will have its thickness larger than that of said slit.
Under such a situation, the thickness limiting member of the prior art must be positioned extremely close to the toner carrying member. This means that an accuracy will be extremely necessary in manufacturing. Furthermore, this means that the narrow slit may be clogged by the toner particles agglomerated due to various causes to form no toner layer.
Therefore, the thickness limiting member must be constructed in the optimum conditions such as materials, shapes, positions and the other so as to form the evenly thin layers of toner on the carrying member.
Even if evenly thin layers of toner can be formed on the toner carrying member, irregularities in development are produced unless the toner carrying member is maintained relative to the image bearing member such as a photosensitive member or the like at a constant distance. Particularly, in such development processes as described in U.S. patent application Ser. Nos. 938,101 and 938,494 and Ser. Nos. 58,434 and 58,435 which are improved by using an AC bias in the above U.S. patent applications, the toner layers are limited to a thickness smaller than the gap between the toner carrying member and the image bearing member so that the variations in said gap will extremely affect reproduced pictures in development irregularity and the like.
In the prior art developing apparatus in which a toner carrying member such as a non-magnetic sleeve is rotatably driven by an image bearing member through a gearing therebetween, excess forces may be applied on the toner carrying member to change said gap or to damage the surface of the image bearing member. This must be also avoided.
The toner particles which have not been used in development must be all collected in a development vessel to prevent them from flying. Flying toner particles will adhere, for example, to the discharging wires of a corona discharging device resulting in charge irregularities, reduction of picture's density or the like.
While various developing devices are utilized in electrographic reproduction machines which are presently used in many offices, it is difficult to effect stable development and thus image-forming over a prolonged period of time in these developing devices unless the above problems are solved.